hair trigger

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A gun trigger adjusted to respond to a very slight pressure.

Century Dictionary and Cyclopedia

1. n. In a firearm, a secondary trigger controlling a safety locking device which secures the chief trigger, by which the piece is fired. The hair-trigger is so adjusted as to be actuated by a very light pressure, and sets free a spring mechanism called the hair, which strikes the tumbler-catch and throws the sear out of a notch in the tumbler.

GNU Webster's 1913

1. n. a trigger so constructed as to discharge a firearm by a very slight pressure, as by the touch of a hair.

WordNet 3.0

1. n. a gun trigger that responds with little pressure